It is known to provide protection in optical networks against line failures, node failures, and the like, by equipping such networks with bypass equipment for bypassing failed components and routing signals to their intended destinations. An example of a prior art network that includes bypass equipment is depicted in FIG. 1. The network includes optical line terminals (OLTs), or nodes, 100 and 200, and a plurality of terminals 100-1 to 100-n, 200-1 to 200-n. The nodes 100 and 200 are bidirectionally coupled to one another through a bidirectional transmission link (L).
The node 100 comprises a plurality of bidirectional communication paths P-1 to P-n and P-1′ to P-n′ that are interposed between an interface (I′) and a WDM multiplexer/demultiplexer (MUX/DEMUX) 106 of the node 100. Bidirectional transponders 102-1 to 102-n are included in the communication paths P-1 to P-n, respectively, and bidirectional transponders 104-1 to 104-n are included in the communication paths P-1′ to P-n′, respectively, of node 100. Although not shown in FIG. 1, the node 200 is assumed to include components which mirror those of node 100.
Bidirectional links L100-1 to L100-n couple an interface IF1 of each of the terminals 100-1 to 100-n, respectively, to node interface (I′), and bidirectional links L200-1 to L200-n couple interface IF1 of each of the terminals 200-1 to 200-n, respectively, to the node 200. Similarly, bidirectional links BL100-1 to BL100-n couple an interface IF2 of each of the terminals 100-1 to 100-n, respectively, to the interface (I′) of node 100, and bidirectional links BL200-1 to BL200-n couple interface IF2 of the terminals 200-1 to 200-n, respectively, to the node 200.
Each of the terminals 100-1 to 100-n and 200-1 to 200-n normally transceives signals through the interface IF1 of that terminal, and transceives signals through the other interface IF2 only in cases where the interface IF1 and/or the link coupled thereto is inactive. Accordingly, the interface IF1 is known to persons skilled in the art as a “working” interface, and the links L100-1 to L100-n and L200-1 to L200-n coupled thereto are known as “working” links. Also, the interface IF2 is known in the art as a “protection” interface, the links BL100-1 to BL100-n and BL200-1 to BL200-n are known as “protection” links, and the transponders 104-1 to 104-n are known as “protection” transponders. Moreover, the terminals 100-1 to 100-n and 200-1 to 200-n are known as “protected” terminals, since they include the protection interface IF2, whereas terminals that do not include a protection interface IF2 are known as “unprotected” terminals.
The so-called protected terminals operate in the following manner. In the event that a failure occurs in the interface IF1 of a terminal 100-1 to 100-n, 200-1 to 200-n, and/or in a link or communication path coupled to that interface, the terminal recognizes the occurrence of the failure and discontinues transceiving signals through the interface IF1. Assuming that the terminal also recognizes that the protection link coupled thereto is active, the terminal resumes transceiving the signals over that protection link through the protection interface IF2. As a result, the failed component is bypassed, and the signals are communicated through the various protection components of the network.
Unfortunately, the above-described network has drawbacks in that it requires the use of many protection components (e.g., transponders 104-1 to 104-n) in the nodes 100 and 200, and those nodes 100 and 200 are generally expensive. Also, the above-described network does not provide any failure protection for unprotected terminals (not shown) that may be included in the network. Accordingly, it would be desirable to provide a network which overcomes the above-described drawbacks, and which provides protection against network component failures for both protected terminals and unprotected terminals. It would also be desirable to provide an optical line terminal that is less expensive than those of the prior art network described above.